Bivalvia is the second largest Class in Mollusca, after Gastropoda. Although it is worth noting that Gastropoda is a vastly larger group, still. Found on the shorelines, right down to the abyssal depths of the ocean worldwide. Bivalvia have a hinged shell, split into two halves. You can use the hinge method to aid identification. Strong adductor muscles are also present to hold the shell closed. Surrounding the body organs is the mantle, which are all held within the shell.
Bivalvia exhibit large gills, adapted to filter feed as well. They siphon feed, allowing water in through an opening, filtering out the organic matter and oxygen with their gills. The gills trap organic particles in mucous, allowing heavier items to fall to the bottom of the cavity. These food items are passed down toward the mouth. Occasionally, Bivalves need to “sneeze” to clean their clogged gills. This waste is called “pseudofaeces”. The exhalant siphon removes the waste water.
Clams have simple lives and therefore have no need for a complex nervous system. Basic eyes are present in the form of Ocelli (left). Ocelli are often found lining the mantle and can detect changes in light. Bivalves generally have chemosensory cells and can also sense light and touch.
Since Bivalves move very little, they reproduce asexually. Most exhibit separate sexes, although some are also hermaphrodites. They release their spawn into the water through the exhalant siphon. Larvae emerge from the fertilised eggs and drift as plankton as they progress through a number of developmental stages. Eventually, they settle on the bottom.
You can find a list of all of the Bivalvia found on The Great Barrier Reef, below.
Order – Fancy science word used to group organisms with similar characteristics
Mytiloida – Mussels
Nuculida – Small Clams
Ostreoida – Oysters
Pectinoida – Scallops
Veneroida – Bivalve Shells